Tea gardens in Assam do not follow the Indian Standard Time (IST), which is the time observed throughout India and Sri Lanka. The local time in Assam’s tea gardens, known as ‘Tea Garden Time’ or Bagantime, is an hour ahead of the IST. The system was introduced during British days keeping in mind the early sunrise in this part of the country.

By and large, the system has subsequently been successful in increasing the productivity of tea garden workers as they save on daylight by finishing the work during daytime, and vice versa. Working time for tea labourers in the gardens is generally between 9 a.m. (IST 8 a.m.) to 5 p.m. (IST 4p.m.) It may vary slightly from garden to garden.

Noted film maker Jahnu Barua has been campaigning for a separate time zone for the north east region.

source: http://en.wikipedia.org/wiki/History_of_tea_in_India